1. Field of the Invention
This invention relates to a method and apparatus for measuring particles in a fluid, and more particularly to a method and apparatus for measuring particle characteristics such as the diameter and size distribution of particles in a fluid, by using a laser beam to irradiate the particle-containing fluid as it flows through a measurement cell, detecting the laser light scattered by the particles in the fluid and determining the characteristics from the intensity of the scattered laser light.
2. Description of the Prior Art
As the density level of large-scale integrated memories and other such semiconductor devices continues to rise, going from 4-megabit to 16-megabit, the high-purity water and chemicals used in the semiconductor fabrication processes have to be of the highest quality, containing no impurities. Controlling fine particles in the pure water and chemicals is particularly important as this has a major effect on LSI yield levels.
One way of measuring fine particles in water and chemicals has been to use a scanning electron microscope. However, using a scanning electron microscope has the drawback of being very costly and lacking real-time capabilities. One widespread solution to this has been to use a particle measurement method comprising irradiating the fluid with a laser beam and determining the particle diameter from the intensity of the laser light scattered by the particles.
The theoretical intensity of the Mie scattering of light from a spherical particle in a fluid can be calculated. It is known that the intensity of light scattered by a particle having a diameter that is smaller than one-tenth the wavelength of the incident laser beam will be proportional to the fifth to sixth power of the particle diameter. It therefore follows that a decrease in the particle diameter is accompanied by a weakening in the intensity of the scattered light, and that to be able to detect such weak light it is necessary to use a detection apparatus that has good signal/noise (S/N) ratio characteristics. Single photon counting is an effective method for detecting weak light.
A conventional apparatus utilizing single photon counting will first be explained with reference to FIG. 5. In FIG. 5, a laser beam from a laser light source 1 passes through a lens 2 which focuses the light onto a particle measurement region 4 of a measurement cell 3. The laser light is scattered by particles which pass through the measurement region 4. The light thus scattered by the particles is condensed by a lens 5 to form an image at a slit 6. The scattered light passes through the slit 6 and impinges on a photomultiplier (PM) 7 whereby the scattered light is converted to electrical signals and output as photoelectron pulses. These output signals are amplified by a preamplifier 8 and are then converted to digital signals by a peak discriminator (DISC) 9 and a pulse shaper 10, and the digital signals output from the pulse shaper 10 are then counted by a pulse counter 11 and the count value is stored in a memory 12 in the form of a time series. The time series data stored in the memory 12 is then analyzed by a processor 13, which uses the intensity values of the scattered light to calculate particle diameter and particle concentration.
Use of the single photon counting method makes it possible to eliminate the dark current and fluctuations in the multiplication factor that are causes of noise in the photomultiplier, providing a three- to five-fold improvement in the S/N ratio compared with the usual analog method. With the single photon counting method, the intensity of the scattered light can be measured by counting the number of photoelectron pulses per unit time interval.
However, the number of photoelectron pulses that can be counted per unit time period is limited by the pulse width and the frequency characteristics of the electrical system constituting the photon counters. Photons reaching the photoelectric surface of the photomultiplier cause electrons to be emitted from the same surface by the photoelectric effect. The electrons emitted from the photoelectric surface are multiplied in number within the photomultiplier by a factor of approximately 10.sup.6. Because of variations in the scanning distance that arise in the course of the electron multiplication in the photomultiplier, each pulse that is output corresponding to the emission of an electron from the photoelectric surface is given a time width.
In the case of side-on type photomultipliers, this time width is usually in the order of 2 ns. Therefore, the emission from the photoelectric surface of electrons at intervals shorter than 2 ns will cause superposing of the photoelectron pulses output by the photomultiplier and make it impossible to count single photons. Even if electrons are emitted at longer intervals than the time width of the photoelectron pulses, the upper count per unit time period is limited by the frequency characteristics of the electrical system constituting the photon counters.
Thus, the three- to five-fold improvement in the S/N ratio compared with analog methods enables smaller particles to be measured with the single photon counting method, with this method the dynamic range is limited by the time width of the photoelectron pulses and the frequency characteristics of the electrical system of the photon counters to a count rate of about 10.sup.8 per second and cannot be used to accurately determine the intensity of high-intensity scattered light from large particles.
In accordance with Mie's scattering theory, the intensity of light scattered by particles in a fluid will also depend on the refractive index of the fluid. Thus, if each fluid (medium) containing the particles has a different refractive index, the intensity of light scattered by particles having the same diameter or refractive index will vary depending on the fluid. As such, in determining particle diameter it is also necessary to take into account the refractive index of the fluid containing the particles which are to be measured.